1. Field of the Invention
The present invention relates to a method, device, and system for processing an optical signal.
2. Description of the Related Art
In an optical fiber communication system that has been put to practical use in recent years, a reduction in signal power due to transmission line loss, coupling loss, etc. is compensated by using an optical amplifier such as an erbium doped fiber amplifier (EDFA). The optical amplifier is an analog amplifier, which functions to linearly amplify a signal. In this kind of optical amplifier, amplified spontaneous emission (ASE) noise generated in association with the amplification is added to cause a reduction in signal-to-noise ratio (S/N ratio), so that the number of repeaters is limited to result in the limit of a transmission distance. Further, waveform degradation due to the chromatic dispersion owned by an optical fiber and the nonlinear optical effects in the fiber is another cause of the transmission limit. To break down such a limit, a regenerative repeater for digitally processing a signal is required, and it is desirable to realize such a regenerative repeater. In particular, an all-optical regenerative repeater capable of performing all kinds of signal processing in optical level is important in realizing a transparent operation independent of the bit rate, pulse shape, etc. of a signal.
The functions required for the all-optical regenerative repeater are amplitude restoration or reamplification, timing restoration or retiming, and waveform shaping or reshaping. Attention is paid to these functions in the present invention to provide an all-optical regenerative repeater in an optical communication system or a signal regenerator at any node point in an optical network by using chirp due to a self-phase modulation (SPM) effect exhibited during propagation of an optical pulse in an optical waveguide structure such as an optical fiber.
The most general one of conventional timing recovery circuits for optical signal regenerating devices is an OE type timing recovery circuit so designed as to once convert an input optical signal into an electrical signal by using a photodetector such as a photodiode, next electrically extract a fundamental frequency, and thereafter intensity-modulate laser light by this fundamental frequency. Such an OE type timing recovery circuit is used for a regenerative repeater in a conventional optical communication system. However, the operating speed of the OE type timing recovery circuit is limited by an electronic circuit for signal processing, so that the bit rate of an input signal is fixed to a low rate.
Another type of timing recovery circuit known in the art is such that in an active MLL (mode-locked laser) incorporating an optical modulator such as an LN (lithium niobate) modulator and an EA (electroabsorption) modulator, the optical modulator is modulated by a fundamental frequency electrically regenerated in a manner similar to the above to thereby regenerate a clock pulse.
On the other hand, an all-optical type timing recovery circuit for optically processing an optical signal on all the stages is also known in the art. Examples of this type timing recovery circuit include a Fabry-Perot laser configured by a saturable absorber, a self-pulsation laser configured by a DFB-LD (distributed feedback laser diode) having integrated reflection and phase modulation functioning portions, and an active MLL for applying AM or FM to a nonlinear medium by signal light.
However, in the case of applying the processing of an optical signal by the conventional timing recovery circuit to wavelength division multiplexing (WDM), it is necessary to prepare a plurality of timing recovery circuits whose number corresponds to the number of WDM channels, causing an enlargement in scale of the device.
It is therefore an object of the present invention to provide a novel method, device, and system for processing an optical signal independent of the bit rate, pulse shape, etc. of the optical signal.
It is another object of the present invention to provide a method, device, and system for processing an optical signal suitable for WDM (wavelength division multiplexing).
Other objects of the present invention will become apparent from the following description.